["Business Ethics, the Environment &Responsibility, EarlyView. ", "\nABSTRACT\nThis study investigates the effect of ethical business practices on a firm's decision to obtain sustainability assurance (SA) and on key SA attributes, namely assurance provider type, assurance level, and assurance scope. Using a sample of non‐financial European firms listed on the STOXX Europe 600 for the period 2012–2019 and applying panel logit regression models, we find that ethically responsible firms are significantly more likely to adopt SA, consistent with stakeholder theory, which emphasises transparency and trust‐building with stakeholders. Ethical practices are not significantly associated with the choice of assurance provider or the level of assurance but are positively related to a broader assurance scope, indicating a substantive commitment to credible and comprehensive reporting. Additional sub‐sample analyses reveal that while both more and less ethical firms, as well as higher and lower ESG performers, tend to adopt SA, the effect is stronger among less ethical and lower ESG firms. This pattern aligns with legitimacy theory, suggesting that SA may also serve as a symbolic tool to manage perceptions and mitigate legitimacy threats. The findings contribute to the literature by jointly drawing on stakeholder and legitimacy perspectives to explain heterogeneity in sustainability assurance adoption and design, and they offer practical insights for regulators, standard setters, and investors seeking to evaluate the credibility of sustainability disclosures.\n"]